The present invention relates to a device for measuring the flow rate of a stream of fluid in a pipe. More specifically, the subject of the invention is a flow rate measuring device comprising a tubular stator intended to be fixed in a pipe so as to be traversed, in use, by said stream, and in which there is axially mounted a rotor which has a central hub from which extends a plurality of blades and which is capable of being operated by said stream, like a turbine, at a velocity variable according to its flow rate, and at least one permanent magnet integral with the rotor and disposed eccentrically with respect to the axis of rotation of the rotor.